With an ITU-T Recommendation G.729, 3GPP AMR, or suchlike speech codec, some of the parameters obtained by analyzing a speech signal are quantized by means of a predictive quantization method based on a Moving Average (MA) prediction model (Patent Document 1, Non-patent Document 1, Non-patent Document 2). An MA-type predictive quantizer is a model that predicts a current parameter subject to quantization from the linear sum of past quantized prediction residues, and with a Code Excited Linear Prediction (CELP) type speech codec, is used for Line Spectral Frequency (LSF) parameter and energy parameter prediction.
With an MA-type predictive quantizer, since prediction is performed from the weighted linear sum of quantized prediction residues in a finite number of past frames, even if there is a transmission path error in quantized information, its effect is limited to a finite number of frames. On the other hand, with an Auto Regressive (AR) type of predictive quantizer that uses past decoded parameters recursively, although high prediction gain and quantization performance can generally be obtained, the effect of the error extends over along period. Consequently, an MA-type predictive parameter quantizer can achieve higher error robustness than an AR-type predictive parameter quantizer, and is used in particular in a speech codec for mobile communication.
Parameter concealment methods to be used when a frame is lost (erased) on the decoding side have been studied for some time. Generally, concealment is performed using a parameter of a frame before an erased frame instead of a parameter of the erased frame. However, in the case of an LSF parameter, parameters prior to an erased frame are gradually modified by gradually approaching an average LSF, or performing gradual attenuation in the case of an energy parameter.
This method is normally also used in a quantizer using an MA-type predictor. In the case of an LSF parameter, processing is performed to update the state of the MA-type predictor by generating a quantized prediction residue so that a parameter generated in a concealed frame is decoded (Non-patent Document 1), and in the case of an energy parameter, processing is performed to update the state of the MA-type predictor using the result of attenuating an average of past quantized prediction residues by a fixed percentage (Patent Document 2, Non-patent Document 1).
There is also a method whereby a parameter of an erased frame is interpolated after obtaining information of a recovered frame (normal frame) that follows the erased frame. For example, in Patent Document 3, a method is proposed whereby pitch gain interpolation is performed, and adaptive codebook contents are regenerated.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI 6-175695    Patent Document 2: Japanese Patent Application Laid-Open No. HEI 9-120297    Patent Document 3: Japanese Patent Application Laid-Open No. 2002-328700    Non-patent Document 1: ITU-T Recommendation G.729    Non-patent Document 2: 3GPP TS 26.091